1. Field of the Invention
The present invention relates to systems and methods for optically profiling and gauging convex surfaces of objects and, in particular, the present invention relates to optical non-contact profiling of the convex surface of objects wherein the object is back lit with converging white light and the resulting shadow image edges are detected and analyzed.
2. Background of Prior Art
There is a need in industry to profile the convex surfaces of objects to ascertain the presence of defects, to aid in the understanding of the manufacturing or shipping processes on the object, or to better analyze the material properties of the object.
For example, aluminum beverage cans are objects having a convex surface around their entire cylindrical sidewall which may be as thin as 0.005 inches. Conventional surface profiling techniques can provide precise measurements of unfilled beverage cans utilizing contact techniques, but such techniques are principally limited to the measurement of the top edge and bottom edge of the sidewall since contact with the center thin sidewalls of the can may cause deflection and, therefore, error in measurement. Hence, a need exists to profile the surface of aluminum beverage cans and to provide accurate metrology of the entire can sidewalls so that important information concerning the measurement and quantification of defects such as dents, may be obtained. Such information provides valuable feedback in understanding, evaluating, and adjusting the manufacturing processes to form the can as well as providing information concerning the material properties of the aluminum in such manufacturing processes.
While the present invention is generally directed towards the surface profiling of aluminum beverage cans, it is to be expressly understood that any convex surface or portion thereof of an object or work piece could be likewise evaluated under the teachings of the present invention.
A number of prior art patented approaches are available for surface profiling such as the following which all relate to an optical reflection technique wherein light, usually a laser beam, is reflected off of the surface of the object. The reflected light is then analyzed. Examples of reflected light approaches are as follows.
U.S. Pat. No. 4,629,319 by Clarke et al. sets forth an invention for the electro-optical sensing of defects such as dents, creases, low spots and flat spots on the surface of sheet metal or plastic panels such as those used on the hoods and fenders of cars, refrigerators and furniture. Clarke directs light onto the surface. The light reflected from the surface impinges upon a retroreflective member to return the reflected light to the surface area to be re-reflected. The re-reflected light is then imaged and carries information as to the nature of the defect.
U.S. Pat. No. 4,326,808 issued to Pryor et al. sets forth an apparatus for determining defects in the outer surface of an elongated object wherein the object to be inspected passes through an aperture of a conical mirror surface. The light is directed onto the mirror surface, reflected by the object, imaged and then analyzed to determine the nature of the defect.
U.S. Pat. No. 4,675,730 issued to Adomaitis sets forth an apparatus for continuously inspecting the surface of a moving object for defects. The surface of the object is illuminated with both specular and/or diffused light of selected wavelengths. The moving surface is rendered momentarily motionless and a plurality of sensors located to view the width of the object detects the presence of the defects. An electronic image of the defect contains gray scale levels that represent varying intensities of the light reflected by the defect. A comparison is then made with a defect free image and, if different, then a freeze frame analysis of the object is made.
U.S. Pat. No. 4,410,278 issued to Makihira et al. sets forth an apparatus for inspecting the outer peripheral surface of a cylindrical object. The light, in slit form, is projected on the surface of the cylindrical object. The reflected light is detected by a photo detector and is quantized at threshold values higher or lower than an average level. The three types of surface defects of a chip, a crack, and a pit are separately detected and identified.
U.S. Pat. No. 4,226,539 issued to Nakagawa et al. also sets forth a system for cylindrical body surface inspection. The cylindrical body is rotated around its axis at a constant speed. Light is directed onto the surface of the body and an optical detector detects the reflected light indicative of a surface condition of a small width baseline which is parallel to the axis of the cylindrical body. A sampling detection repeats as the body rotates to scan the entire surface of the cylinder.
U.S. Pat. No. 4,162,126 issued to Nakagawa et al. sets forth a camera system which senses diffused reflected light from the surface of an object in order to analyze the reflected light wherein a threshold level is used so that surface defect patterns such as a broken cavity, a pit, or a crack pattern can be selectively discriminated.
All of the above represent prior approaches for surface profiling and gauging involve a non-contacting system like the present invention. However, each of these approaches analyze light which is reflected from the surface and they are suitable for evaluating concave surfaces. The present invention does not reflect light nor does it analyze the reflected light.
The following prior art approaches set forth in inspection techinques which analyze the shadow of the object being inspected.
U.S. Pat. No. 4,576,482 issued to Pryor sets forth an apparatus for determining accurate dimensions of individual work pieces. The apparatus is a non-contacting system wherein a collimated or semi-collimated light source illuminates at least one edge of the work piece with parallel light rays so that a lens can form an image of the illuminated edge. This image provides an average shadow over an area of the edge. An array of photosensitive elements such as photodiodes produce an electrical signal in response to the light impacting thereon. The edge image as sensed by the photodiode array can then be analyzed to provide a determination of a dimension such as length, squareness, curvature and the like.
The 1972 patent to Hemsley (U.S. Pat. No. 3,666,885) utilizes a strobe for directing a short duration light pulse onto the object to form a shadow image of the object in a camera. The camera is modified for single line scanning and produces a line waveform of the shadow image. The Hemsley approach is adaptable for either hot or cold objects being scanned. Hemsley utilizes a referenced object to determine the initial measurement and then compares the object being inspected to the measured value and records any differences in the line waveforms.
The 1984 patent to Forbes (U.S. Pat. No. 4,465,937) utilizes a light source mounted in a scanning head that is rotated about the object and which can be advanced along the length of the object to provide data pertaining to the entire peripheral surface of the object. Forbes utilizes a light source that provides a beam of light having a width greater than the width of the object being scanned so that as the light source is rotated around the object, deviations in the shadow's edge can be sensed by photo sensors and determined.
The 1984 patent to Daudt (U.S. Pat. No. 4,476,533) pertains to a non-contact optical gauger for measuring hot glassware articles while being manufactured. The system makes specific measurements of height, perpendicular, neck diameter, and height variation of the glassware article. Articles falling outside predetermined measurements are rejected.
The above prior art approaches for surface profiling involve a non-contacting system utilizing the shadow image from light projected onto the object. The present invention is similar to these approaches but substantially improves upon them. The present invention optically creates a plurality of knife-edge shadows of the convex surface of an object without the generation of error causing glints as the surface is rotated and then digitally analyzes the edges. The present invention utilizes converging white light in a reduced field of view and is automatically capable of creating from the plurality of edges a dense array of topological data having 2000 to 5000 individual data points per inch for the surface of an object being profiled without distorting the surface of the object through contact. The cylindrical telescope of the present invention narrows the field of view in the horizontal direction of the shadow image to magnify the presence of defects. This increases the accuracy of sub-pixel analysis of the present invention.